Our invention relates to the electrical protection of communication equipment and, more particularly, to improve arrangements for current and voltage overload protection.
Communication lines may carry foreign electrical energy from, for example, lighting strikes and power transmission line faults. In order to safeguard communication equipment connected to such lines, prior protection circuits may include carbon blocks, gas tubes, fuses and heat coils. Carbon blocks, however, have a limited lifetime and may become electrically noisy. Gas tubes are comparatively costly. Fuses and heat coils may be slow in operation and are not resettable.
More recently, switching or crowbar type protection circuits have been developed which include semiconductor elements. Such circuits, however, may not be adapted to the specialized protection requirements of communication circuits. A communication protector should be resistant to latching due to the constant DC supplied by a central office. In addition, if foreign voltage appears between one conductor and ground of a two conductor communication line, it is also desirable to provide balanced protection by grounding both conductors. Further, modern construction trends toward out-of-sight equipment installation urge that a protector be physically compact.
It is therefore an object of the invention to provide a reliable, compact and economical protection circuit that operates rapidly and is automatically resettable.